1. Technical Field
The present invention relates to a part of a through-hole forming method for a piezoelectric substrate used in a piezoelectric device, and a method of manufacturing a piezoelectric device using the part of a through-hole forming method and a piezoelectric device manufactured thereby. More specifically, the invention relates to the piezoelectric substrate formed with an excitation electrode on a first surface, and the part of a through-hole forming method suitable for the piezoelectric device provided with the piezoelectric substrate and the piezoelectric device.
2. Related Art
The piezoelectric device used for an oscillator, a filter, a sensor or the like has been promoted to be compact and thin based on a downsizing of a device to which the piezoelectric device is mounted. The smallest piezoelectric device among what are currently under development is one having a, so-called chip size package (CSP) structure in which a package including a piezoelectric element is manufactured substantially the same size as the piezoelectric element. An example of the piezoelectric device of such structure is, among others, a surface acoustic wave (SAW) device. This example is described in JP-A-8-213874. The SAW device described in JP-A-8-213874 includes, as illustrated in FIG. 9, a piezoelectric substrate 2 formed with an excitation electrode 3 and leading electrodes 4 on a first surface thereof and a covering body 5 for covering the first surface of the piezoelectric substrate 2. More specifically, the covering body 5 is provided with a concave portion 6 at a position opposing to the excitation electrode 3 and part of a through-hole 7 at positions opposing to the leading electrodes 4, and the piezoelectric substrate 2 and the covering body 5 are bonded by means of anodic bonding. The through-hole 7 of the covering body 5 after bonding is filled with conductive material, and mountable exterior electrodes 8 are formed around the part of a through-hole 7. In the piezoelectric device 1 of such a structure, a mounting area of the element equals to that of the device, resulting in improvement of a downsizing of the piezoelectric device.
The piezoelectric device having the structure as stated in JP-A-8-213874 is considered as being suitable for downsizing and thinning of the piezoelectric device. However, the SAW device having the structure as stated in JP-A-8-213874 includes drawbacks as stated below.
First, since the piezoelectric substrate serving as the covering body is formed on one surface with the concave portion and the part of a through-hole, there is a high possibility for foreign matters to be attached to and remaining at the concave portion and the part of a through-hole while cleaning the substrate or the like. Second, the piezoelectric substrate and the covering body are bonded through a thin metal film formed around an edge portion of the piezoelectric substrate, such that there is a possibility that a gap may be created between each of the leading electrodes and each of the part of a through-hole. In this case, there arise possible troubles that conductivity between the external electrodes and the leading electrodes can not be established and an airtight sealing of an interior space may be degraded, thereby inviting a possibility of an undesirable invasion of gas or dust from the gap into the interior space.